Datasheet

Analog Integrated Circuit Device Data

79 Freescale Semiconductor

MC34708

Functional Block Description

from address 0 to the stop address programmed in ADSTOP. By default, the ADCONT is set low (disabled). In the continuous

mode, the ADHOLD bit will allow the software to hold the ADC sequencer from updating the results register while the ADC results

are read. Once the sequence of A/D conversions is complete, the ADRESULTx results are stored in 4 SPI registers (ADC 4 -

ADC 7).

7.7.3 Dedicated Readings

7.7.3.1 Channel 0 Battery Voltage

The battery voltage is read at the BATTISNSN pin on channel 0. The battery voltage is first scaled as V(BATT)/2 to fit the input

range of the ADC.

7.7.3.2 Channel 1 Battery Current (Optional)

Battery current is only valid after a battery voltage reading. The current flowing into and out of the battery can be read via the

ADC by monitoring the voltage drop over the sense resistor between BATTISNSN and BATTISNSP.

The voltage difference between BATTISNSN and BATTISNSP is amplified to fit the ADC input range as V(BATTISNSP -

BATTISNSN)*15. Since battery current can flow in both directions, the conversion is read out in 2’s complement. Positive

readings correspond to the current flowing into the battery, and negative readings to the current flowing out of the battery.

The value of the sense resistor used determines the accuracy of the result, as well as the available conversion range. Note that

excessively high values can impact the operating life of the device due to extra voltage drop across the sense resistor.

If battery current sense is required, add a 20 m resistor between the BATTISNSN and BATTISNSP terminal, as shown in

Figure 19.

Table 70. Battery Voltage Reading Coding

Conversion Code

ADRESULTx[9:0]

Voltage at Input ADC in V Voltage at BATTISNSN in V

1 111 111 111 2.400 4.800

1 000 010 100 1.250 2.500

0 000 000 000 0.000 0.000

Table 71. Battery Current Reading Coding

Conversion

Code

ADRESULTx [9:0]

Voltage at input

ADC

in mV

BATTISNSN–BATTISNSP in mV

Current through

mOhm in mA

Current Flow

0 111 111 111 1200.00 80 4000 To battery

0 000 000 001 2.346 0.156 7.813 To battery

0 000 000 000 0 0 0 -

1 111 111 111 -2.346 -0.156 7.813 From battery

1 000 000 000 -1200.00 -80 4000 From battery